We observe that the weight given above for Achem, following
all the metrologies, is based on old data, the accuracy of which one may
doubt, especially since this weight does not agree in any way with those of
other parts of the island of Sumatra, such as Bencoulen and Fort
Marlborough, whose weights were verified in London based on
standards received directly in 1821.

Odd as this inconsistency seems, it appears to be genuine:

The most established weight in trade, is the tial or tael,
which differs however in the northern and southern parts of the island, being in
Natal twenty four penny weights, nine grains [37.9 grams], and at Padang,Bencoolen and elsewhere, twenty six penny weights, twelve grains [41.2 g].

William Marsden.The History of Sumatra… Second Edition.
London: Printed for the author, 1784.Page 136.

Kelly (1835), who had the
active cooperation of the British East
India Company and made a special effort to ascertain East Asian weights accurately,
also states “the tale differs in the Northern and Southern parts of the island” (page
112). He gives the following magnitudes: